1. Field of the Invention
The invention relates to the field of electrically programmable read-only memories, particularly those employing floating gate memory devices.
2. Prior Art
Metal-oxide-semiconductor (MOS) electrically programmable read-only memories (EPROMs) frequently use memory cells that have electrically isolated gates (floating gates). These floating gates are typically completely surrounded by oxide and formed from a polycrystalline silicon (polysilicon) layer. Information is stored in the memory cells or devices in the form of charge on the floating gates. Charge is transported to the floating gates by a variety of mechanisms such as avalanche injection, channel injection, tunnelling, etc., depending on the construction of the cells. The cells are erased generally by exposing the army to ultraviolet radiation. An example of these cells can be found in U.S. Pat. Nos. 3,500,142; 3,660,819; 3,755,721; and 4,099,196. In some cases these cells am electrically erasable (EEPROM). An example of such a cell is shown in U.S. Pat. No. 4,203,158.
The invention of the present application is used with an EPROM cell particularly one which is electrically erasable, referred to as a "flash" EPROM cell. The cell used in conjunction with the present invention is described in copending application Ser. No. 253,775, entitled Low Voltage EEPROM Cell, filed Oct. 5, 1988, and assigned to the assignee of the present invention.
It has been known for many years that when EPROM cells am used in a memory array, circuitry is sometimes required to electrically isolate the devices, one from the other. This may be needed, for example, to permit the reading of one cell without interference from adjacent cells, or for instance, to permit the programming of a cell without disturbing the programming of another cell. For examples of this, see U.S. Pat. Nos. 3,728,695 and 4,698,787.
A variety of sense amplifiers are used in the prior art to sense the state of cells. One category of these amplifiers relies on a reference potential developed by a dummy or reference cell. An example of this type of sense amplifier is shown in U.S. Pat. No. 4,223,394.
As will be seen, the present invention provides isolation of sorts, in a flash EPROM memory array. However, with the present invention, this isolation is used for the reference cells or devices used in the array as part of the sensing circuitry.